MAASTRICHT, Netherlands — Underwriters must maintain control of their core business functions as brokers infringe on their traditional territory through insurance sidecars and other facilities, a panel of insurance executives said Tuesday.

The new facilities, including the London-based vehicle established by Aon P.L.C. and Berkshire Hathaway Inc. earlier this year, blur the lines in the traditional insurer, broker and buyer relationship, they said, speaking at the Federation of European Risk Management Associations' annual forum in Maastricht, Netherlands.

An Aon executive, however, said the facility adds capacity to the market and has been well-received by the brokerage's clients.

For insurers to offer value to risk managers as the world grows increasingly volatile, they must offer stability and continuity, said Axel Theis, CEO of Allianz Global Corporate & Specialty A.G. in Munich.

To do that, he said, insurers must maintain control of their core services: underwriting, pricing, risk consulting and claims management.

The facility offered by Aon and Berkshire Hathaway offers guaranteed capacity for business in which Lloyd's of London syndicates participate. Willis Group Holdings P.L.C. has indicated that it is establishing a similar facility.

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The broker facilities do not offer the same range of services that insurers provide corporate policyholders, such as working with captives as well as offering international claims services and risk engineering capabilities, said Thomas Hürlimann, CEO global corporate at Zurich Insurance Co. Ltd.

The sidecar facilities just add more capital to the market, he said.

The broker facilities highlight the different perspective of insurers and brokers, said Peter D. Hancock, CEO of AIG Property Casualty, a unit of American International Group Inc. As providers of insurance capacity, insurers have to offer long-term stability and consistency, whereas brokers are “inherently more agile” and can change their business more frequently, he said.

The new facilities may blur the lines between brokers and underwriters “in a parasitic way,” so they are “freeloading off underwriters” who put their own capital at risk over the long term, Mr. Hancock said.

But Robert Brown, CEO of Aon Risk Solutions EMEA in London, defended the Aon facility, saying it has brought new capacity to the market. “It's very securely rated capital as well, so in terms of claims paying, it will be far more effective and faster,” he said during another panel session at FERMA.

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However, rival brokerage executive James Twining, group commercial director at Jardine Lloyd Thompson P.L.C., said insurance sidecars are “free riders” on Lloyd's underwriting that put a huge amount of financial pressure on smaller Lloyd's syndicates.

“They are not in the best interests of the client or the market,” Mr. Twining said.

Still, insurers must accept that “these facilities are not going to go away,” said Richard Ward, CEO of Lloyd's of London during the insurer panel. “As a market, our response needs to be focused on risk selection and claims management,” he said.

But the new facilities likely will be short-lived, said Mike McGavick, CEO of XL Group P.L.C. Insurers providing the capacity will abandon the facilities as their experience deteriorates, he said.

“If you do provide dumb capacity, the capacity over time will be abused,” Mr. McGavick said. As a result, the original capacity providers will exit that part of the market and only the “copycats” will be left, he said.